Codeforces 107B Basketball Team

 1 // File Name: 107b.cpp
 2 // Author: darkdream
 3 // Created Time: 2015年03月24日 星期二 21时40分11秒
 4 
 5 #include<vector>
 6 #include<list>
 7 #include<map>
 8 #include<set>
 9 #include<deque>
10 #include<stack>
11 #include<bitset>
12 #include<algorithm>
13 #include<functional>
14 #include<numeric>
15 #include<utility>
16 #include<sstream>
17 #include<iostream>
18 #include<iomanip>
19 #include<cstdio>
20 #include<cmath>
21 #include<cstdlib>
22 #include<cstring>
23 #include<ctime>
24 #define LL long long
25 
26 using namespace std;
27 int n , m , h ; 
28 int a[1005];
29 int main(){
30     scanf("%d %d %d",&n,&m,&h);
31     int sum = 0 ; 
32     for(int i = 1;i <= m;i ++)
33     {
34        scanf("%d",&a[i]);
35        sum += a[i];
36     }
37     a[h] -- ; 
38     n -- ;
39     sum -- ;
40     if(sum < n )
41     {
42         printf("-1\n");
43         return 0 ; 
44     }
45     int r1 = sum;
46     int l1 = (sum-a[h] + 1);
47     int l2 = sum - a[h] - n +1 ; 
48 //    printf("%d %d %d\n",r1,l1,l2);
49     double ans = 1.0;
50     for(int i = l1 ;i <= r1;i ++)
51     {
52         ans = ans * (double)(l2+(i-l1))*1.0/i; 
53     }
54     printf("%f\n",1.0- ans);
55 return 0;
56 }